Foundation slab thermal insulation

Hello,

A question,

We will be building soon, and we are about to submit the building application.

In our new development area, there are already a few finished poured floor slabs. On some floor slabs, insulation is applied on the front side (on the sides), I think Styrodur, while on some the insulation is completely missing, you only see concrete.

Is the insulation necessary, or is it advantageous? I just wanted to have a few brief pieces of information before I call the site manager and ask strange questions!

Thank you

Hello,

the insulation on the outside is certainly always necessary when we mentally think about a single-family house. However, the insulation can also be applied at a later time. I would be more concerned if the perimeter insulation is applied too early, for example as formwork during pouring.

Without the perimeter insulation of the floor slab against the ground, a corresponding thermal bridge would indeed arise there. Whether this is strictly necessary due to regulations and the like, I do not know.

I would be more concerned if the perimeter insulation is applied too early, e.g. as formwork when pouring.

hm I have now seen dozens of construction sites where the insulation was applied before pouring, are you saying that is wrong?

Well, let's put it this way. When we were in that phase, I spent a long time dealing with it. We had the problem that our base slab became a bit too large and it was chiseled away, which partly exposed the reinforcement, and the issue of sealing was also examined more closely in the process. The question is: How should the plinth area be properly sealed if perimeter insulation is already installed?

hm I have now seen dozens of construction sites where the insulation was applied before pouring, are you saying that is wrong?

For intermediate ceilings this is correct.
For a floor slab it depends on the sealing system and is unusual because water can penetrate behind the insulation through the slots and gaps between the insulation boards.

There are different sealing systems, one possibility is for example:
- first pour the floor slab using formwork
- build walls set back inside (horizontal barrier)
- create a fillet between floor slab and wall (important)
- apply sealing slurry (in areas where higher moisture is expected)
- apply thick coating (up to the base)
- install perimeter insulation from bottom to top overlapping
- dimpled membrane